The recent refugee crisis in Europe has brought the Schengen Agreement into the news. This obscure agreement, which took full effect in 1995, removed border controls to allow the free movement of citizens of participating European countries. Its scope has been expanded: 26 E.U. nations today fall into the Schengen zone. Some member nations are now tightening border controls as a means of managing the influx of immigrants.

Regardless of that development, the Schengen Agreement affects citizens of the United States and Canada traveling to Europe. Those considering a trip to the E.U. should understand the rules and the affect the agreement might have on travel plans.

Although it sanctioned freedom of movement within the Schengen zone countries, the agreement also imposed 90-day visa limits for most outside visitors to the 26 countries within its borders. During any 180-day period, visitors are allowed to spend no more than 90 days in all Schengen countries combined. Visitors can't spend 90 days in Portugal and then 90 days in France, for example. Unlike in parts of Asia, the clock does not reset by leaving on day 90 and returning a day later.

The penalty for exceeding the 90-day stay can be a 500 euro fine and prohibition from entering the Schengen zone for up to three years.

Even travelers arriving in Europe for a business trip or short vacation should be mindful of the Schengen Agreement. Passports must be valid for at least three months after entry date or you may be denied admittance.

Despite our initial skepticism, we can attest to the stringency of this little-known regulation. During our round-the-world journey, we were directed to a "Non-Schengen" immigration line in Malta, our first European destination on that trip. Whenever we exited or re-entered the Schengen zone, our passports were rigorously examined to ensure we hadn't overstayed our 90-day limit.

For additional information about member countries and requirements, go to: http://travel.state.gov/content/passports/en/go/schengen-fact-sheet.html.

Philadelphia natives Larissa and Michael Milne have been traveling the world full-time since 2011. Get more travel tips on their blog, www.ChangesInLongitude.com.